Nixtamalization is an age-old process that consists of cooking corn grain in alkaline water. The cooked grain is then ground and the product known as “mass” is used to make tortilla, an elemental component of the Mesoamerican peoples' diet and which use has been extended throughout the world. In an industrial scale, the mass is dried and ground, and is commercialized as “nixtamalized” corn flour. In any case, both the water used for the alkaline cooking and the water used for washing the cooked grains, are known as nejayote: an effluent rich in organic matter, composed by both suspended solid particles and solids in solution. Unlike the water used to soak the corn generated by the starch industry, nejayote's composition is highly complex, since it contains the resulting products of the alkaline hydrolysis of the pericarp and part of the endosperm of the grain.
The relationship, composition and nature of the components of nejayote formed or released during nixtamalization depend on several different process parameters, including the variety of the corn grain, hardness of the endosperm, quality and integrity of the grain itself and concentration of the lime (Rosentrater, 2006, Waste Management 26, 284-292), as well as temperature and time of cooking and soaking. In the same review, 12 reports are mentioned in which a range of weight loss during nixtamalization can be established between 0.5 and 14.5% of dry weight, which would mean a range between 2500 mg to 50000 mg of total solid components per liter of nejayote. Of these, suspended solid particles range from 2350 to 5500 mg/L and the rest is soluble material formed by carbohydrates of different chain length, phenolic components and calcium salts. The high concentrations and complexity of the organic material, high pH and temperature make nejayote a highly contaminant effluent and hard to treat by conventional methods.
For example, the compact plants for the treatment of residual water have had little success due to the fact that the presence of elements that inhibit microbial growth in nejayote, and its high organic burden, increase greatly the hydraulic residence times and present inlay problems because of the high contents of calcium. Furthermore, although oxidation lakes are a technically viable option, in the majority of cases the big nixtamal-processing companies and medium-sized and small-sized mills are usually in urban centers. This location does not allow the installation and commissioning of the lakes because of lack of space and bad odor, which would upset the residents of the surrounding areas. Other strategies to make use of this effluent (as described by Viniegra and Ramirez, 2007, MX/A/2007/010310), consist on isolating the suspended solid components of nejayote and reincorporating them to the corn mass with the further addition of the liquid to the nixtamalization process. This, however, results in the final product with unpleasant taste and smell, which are not accepted by the consumer; so this strategy has had little success in its application. In laboratory studies that we have conducted, we have found out that the unwanted organoleptic properties of smell and taste are mainly originated from the phenolic compounds found in nejayote, at pH and temperature of nixtamalization. For example, it was found that a phenolic compound known as ferulic acid is transformed in 4-vinyl-guaiacol, and that orto-oxidative coupling reactions of phenols are promoted. Due to the absence of efficient technologies for the treatment of nejayote, different strategies have directed the efforts to the development of alternate technologies to the traditional nixtamalization process with the purpose of reducing or completely eliminating the generation of nejayote (Wilkinson y Short, 1998, EP0883999; Sánchez y de la Cámara, 2004, WO/2004/023892, 2004; Figueroa y col., 2006, MX/A/2005/011797; Bejarano y col., 2009, WO/2009/143416). Nevertheless, said technologies have been unattractive to be applied due to their high operation and investment costs.
Recently, Asaff et al., 2013 (MX/a/2013/000943) developed a unique technology to eliminate any particles or suspended solids from nejayote, process in which one of its applications would allow the integral treatment of nejayote with the use of membrane technology, an increasingly common technology for the treatment of residual water and other diverse industrial processes. For example in Potter's patent, U.S. Pat. No. 5,707,524, a process and a system for the treatment of residual water is described, for different carbohydrate-containing industries as a fermentable material for yeast production. In the patent, the use of ultra-filtration membranes to concentrate fermentable material and generate water to be reused or discarded to the sewage system with a minimal quantity of contaminants is described. However, recent studies that we have been conducting show that in both cases a series of limiting factors exist, which would prevent their industrial application at reasonable costs. In the first case of elimination of suspended solids, the technology described states that it is necessary to use phosphoric acid or a mixture of phosphoric acid or any of its salts with another strong acid to lower the pH to a range of 4 to 6.5, the optimum range of action of certain kind of enzymes, and on the other hand to give the ionic strength to favor the flocculation of the suspended material. The use of phosphoric acid or any of its salts causes the formation of a buffer solution that requires an increased use of acids to achieve the desired pH. Phosphoric acid, comparatively to other strong inorganic acids, has the highest price in the market, which added to the greater use of acid, increases the cost of operation. On the second case, which uses ultrafiltration membranes, our studies showed that nejayote contains some components that gradually get adsorbed onto the membrane material, causing, after some time of use, pore saturation and obstruction, as well as deterioration due to chemical reactions of positively charged compounds and the material of the membrane. Thus, usage of any kind of membrane, be it ultrafiltration, nanofiltration or reverse osmosis, in the treatment of nejayote obtained or conditioned in the parameters described in the application MX/a/2013/000943, is not technically feasible.
Some other described technologies claim to solve the problem indirectly through the recovery of the products of high value contained in it, like feruladed pentosans or feluric acid, with which its total organic matter decreases. Carvajal-Millan et al., 2005, MX 278768 describe a process of ferulated pentosans that consists of adding etanol to a solid-free nejayote and adjusting the pH to 5, with the intention of precipitating medium and long chain carbohydrates. However, its application on industrial systems is very limited because of the costs involved in the recovery of alcohol, which has a ration of at least 2 to 1 when compared to nejayote. Furthermore, the recovered final product is heterogeneous because it contains carbohydrates of different length as well as starch, and the recovery cost is high because of the quantity of alcohol needed. On the other hand, in the patent of Asaff et al. 2005, MX 278768, they describe a process for the recovery of ferulic acid from nejayote by means of adsorption/desorption processes. That system allows us to keep the phenolic fraction and leave the major organic fraction in solution, composed of carbohydrates of different nature. In both patents, the material successfully removed varies between 5 and 20%, so for the most part, the problem of nejayote is still latent.
Since there are no viable alternatives for the treatment or handling of nejayote to this day, in most cases this effluent is discarded in the municipal sewage, or natural water currents with little to no treatment. This means that the activity of the industry of nixtamal, mass and tortilla has a negative impact in the environment, leaving it vulnerable to fines before environmental regulations get more and more strict. For this reason, it is urgent to generate effective alternatives for the integral treatment of nejayote, facilitating its reuse or discharge in an environmentally safe way; and thus promoting the establishment of socially responsible companies, which have policies of care and conservation of the environment.